1. Technical Field
The invention relates to the field of communications signal processing and switching methods and apparatus and in particular to a switching system in which respective subscriber locations or line terminating groups lie on a ring-shaped bus to which are connected at least two switching signal processors for establishing a call connection.
2. Description of the Prior Art
A switching system is already known, U.S. Pat. No. 4,032,899, in which a bus links a plurality of line terminating modules and a plurality of switching processor modules. Line used herein refers to both a line to a subscriber and to a trunk to another telephone switch. Each line terminating module (trunk terminating module) includes a group of transmission lines. The switching processor modules are directly linked to a line terminating module or to a group of transmission lines. Each switching processor module contains look-up tables in which the pre-determined allocations between each switching processor and its associated group of transmission lines are stored. In addition there is stored in each switching processing unit interconnect information on those switching processor modules which are allocated to all remaining transmission lines.
The system is operated to perform both circuit switching and store-and-forward switching of data packets. Circuit switching is accomplished by a central node control module by establishing a direct connection between line terminating modules, without the intervention of switching processor modules. For the transmission of data packets between two transmission lines, a virtual connection is established by the central node control module, and a pair of switching processor modules are assigned respectively to the sending transmission line and the receiving transmission line. After establishment of the virtual connection, the data packets are transmitted from the sending transmission line to the associated switching processor module and stored in a buffer therein. The first module then sends a clearance request to the switching processor module assigned to the receiving transmission line. When reception has been cleared, the first switching processor module retransmits the data packets to the line terminating module to which the receiving transmission line is connected. The second switching processor module is not involved in this transmission. This means, however, that a disturbance in the first switching processor module can very easily disrupt the establishment of the connection.
There are other ring-shaped bus systems known e.g. U.S. Pat. No. 2,986,602; "IBM Technical Disclosure Bulletin", Vol. 7, No. 7, Dec., 1964, pages 592, 593 in which communications between subscriber locations or between two groups of subscriber locations through a single central computer may be established. Mere again, however, the desired dependability during the establishment of communications is not available.